A low-temperature fluid pump for supplying a fluid having a lower liquefaction temperature than air, such as liquid nitrogen, is mostly used for feeding a fluid of a saturated vapor pressure (approximately 77 K at atmospheric pressure), and is often configured to have a low required net positive suction head (required NPSH) for the purpose of preventing vaporization of the fluid by negative pressure or the like. For this reason, it is preferred that a check valve used in a pump have a low flow rate resistance (high flow rate coefficient).
On the other hand, a positive displacement pump such as a bellows pump described in Patent Literature 1 is configured to increase the speed of the pump stroke or, in other words, to reduce the time required in one stroke, so that reduction in size and weight of the pump apparatus, especially the weight of the pump support member or bellows operating shaft, can be realized, as well as reduction of the impact of the heat generated in the drive unit. Therefore, in order to achieve high-speed pump stroke, the time required by a behavior of a check valve used needs to be reduced. Specifically, in case of the poppet check valve described in Patent Literature 2, it is required to reduce the time it takes for the valve element to return by its own weight in a closure stroke. Moreover, a delay of the closure timing leads to a reverse flow of the fluid at valve closure, and the impact of a water hammer caused by the reverse flow is not negligible.
As a way to resolve a delay in valve closure, generally there is means for applying spring force in the return direction of the valve element or reducing the degree of opening of the valve element at valve opening (check valve with a spring, etc.). There is also means, such as the one described in Patent Literature 3, for forcibly closing the valve by using external force of a cam, a solenoid, or the like. However, applying spring force or reducing the degree of opening leads to a relatively low flow rate coefficient of the valve, which means that the valve needs to be enlarged to obtain a required flow rate coefficient, increasing the size of the pump itself. Furthermore, the configuration of forcibly closing the valve makes the mechanism complicated. The conditions of a low-temperature use environment and thermal insulation need to take into consideration, but such requirement makes it difficult to design the pumping mechanism.
The behavior of the valve element of a poppet check valve, on the other hand, is known to have a great impact of the force acting on the valve element due to the momentum of the fluid, as can be seen in the model shown in FIGS. 12A and 12B. FIGS. 12A and 12B are schematic diagrams for explaining the force acting on the valve element of the poppet valve. The force applied to the valve element is not only associated with the differential pressure ΔP (=P1−P2) between the upstream pressure (P1) and the downstream pressure (P2) in the valve element but is also associated with the momentum of the fluid. The force acting on the valve element of the poppet valve is obtained by the following equation according to the model shown in FIGS. 12A and 12B.F=A·ΔP+ρ·Q·(V0−V·cos θ)  (1)V=C/A·√(2/ρ·ΔP)  (2)
In these equations, A represents the cross-sectional area of the pipe (=π·d2/4), d represents the diameter of the pipe, ρ represents the fluid density, Q represents the flow rate (=V0·A), V0 represents the flow velocity at the upstream, V represents the flow velocity at the valve portion, C represents the flow rate coefficient, and θ represents the angle formed by the tapered surface of the valve element and the axial line.
The first term on the right side of the equation (1) represents the force caused by the differential pressure ΔP between the upstream and the downstream, and the second term represents the force caused by the momentum of the fluid. In the structure of a self-weight operated valve, when the lift distance of the valve is substantially great with respect to the flow rate, V≈V0 is established. Therefore, as a result of substituting the equation (2) and making adjustments, the equation (1) becomes as follows.F=1/2·ρ·A·V02·1/C2+ρ·A·V02·(1−cos θ)  (3)
The first term on the right side of the equation (3) represents the force caused by the differential pressure ΔP between the upstream and the downstream, and the second term represents the force caused by the momentum of the fluid. Compared to a valve that only has the action of the differential pressure ΔP and the same flow rate coefficient with respect to the lift distance, the level of the force that pushes up the valve element is higher by the level of the force represented by the second term, even when the flow rate is the same. This increases the time it takes for the poppet valve to drop from its position at the maximum lift distance to the valve-closed position by its own weight, creating a delay of the closure timing.